Guarding your cases 2019

Guarding Your Cases

Changing the Guards

Below is a function that uses guards. However, guards are so confusing that we aren’t quite sure how to write it. So, we have presented 4 different possible solutions.

Which is correct? What are the issues with the other examples? What errors and/or warnings would Haskell return for those that are wrong?

• Example 1:

    traffic :: String -> String
    traffic light
        | light == "green" = "go!"
        | otherwise = "slow down."
        | light == "red" = "stop!"
  

• Example 2:

    traffic :: [Char] -> [Char]
    traffic light
        | light = "green" == "go!"
        | light = "red" == "stop!"
        | otherwise == "slow down."
  

• Example 3:

    traffic :: String -> String
    traffic light
        | light == "green" = "go!"
        | light == "red" = "stop!"
        | otherwise = "slow down."
  

• Example 4:

    traffic :: String -> String
    traffic light
        | light = "green" = "go!"
        | light = "red" = "stop!"
        | otherwise = "slow down."
  

The Problem

Today, we’ll be creating your own miniature program that allows you to play Rock-Paper-Scissors! To start off, here are the type you’ll need:

data Move = Paper | Rock | Scissors deriving (Eq, Show)
data Result = Win | Draw | Lose deriving (Eq, Show)

(The first part of the problem is writing a function which takes a move as input and finds the move which beats it). We have written most of the function but some parts are missing:

beats :: Move -> Move
beats move
    | move == Paper = _
    | __ == ___ = _
    | otherwise     = _

Now write a function which returns the result to a set of moves. Here is a framework:

score :: Move -> Move -> Result
score playerA playerB ...

Case Example

We now move onto cases! They are quite similar to guards but have a few key differences. Instead of checking boolean expressions, cases use pattern matching: they see if the case matches the same expression as the pattern.

We have tried to rewrite the traffic function using cases. But again, we aren’t entirely sure how to do it properly. Can you identify the correct function and what errors/warnings will appear for the other?

• Example 1:

    traffic :: String -> String
    traffic light = case light of
        "green" = "go!"
        _       = "slow down."
        "red"   = "stop!"
  

• Example 2:

    traffic :: String -> String
    traffic light = case light of
        "green" -> "go!"
        "red"   -> "stop!"
        _       -> "slow down."
  

• Example 3:

    traffic :: String -> String
    traffic light = case light of
        "green" = "go!"
        "red"   = "stop!"
        _       = "slow down."
  

The Case with Guards

Congratulations, you can now play playgroud rock-paper-scissors with your friends. However a challenger approaches, and they only trust cases. Try rewrite both beats and score with cases!